According to the developers of King Camera (which is my go-to camera app), there is no way to directly control the aperture or shutter speed of the iPhone's camera. The only option is to use the focal and/or exposure points of the various camera apps that have them to control the exposure. Camera+ is a very good camera app. But if the negative reviews turn you off, you may want to look at King Camera.

David
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It is impossible for a man to learn what he thinks he already knows
--Epictetus